The present invention relates generally to antennas for satellite-based communication systems, and more particularly to antenna feed assemblies capable of configuring the communication ports of an antenna at selected polarizations.
In the past few years, there has been a significant increase in the number of satellite-based communication systems. As with other types of communication systems, however, there is a limited amount of bandwidth to handle this increase. For this reason, a technique known as frequency reuse is typically implemented. In this technique, signals used in communication with a satellite, (such as two receive signals or a transmit and receive signal), are oriented in polarization planes with respect to each other, so that both signals can reside on the same channel, (one in each plane). As such, the channel is used for communication of two signals as opposed to just one, thereby increasing the amount of information that may communicated on each channel of the frequency band. The signals may be either at the same polarization, (co-polarized), orthogonal to each other, (cross-polarized), or at a predetermined polarization difference.
Antennas used in frequency reuse applications, typically include a feed assembly for coupling either two receive waveguides or a transmit and receive waveguide to a common feedhorn, depending on the requirements for the antenna application. The orientation of the ports of the common waveguide for connecting the receive and transmit waveguides to the feed assembly determine the polarization for each waveguide. As an aid to understanding this concept, FIGS. 1A and 1B respectively illustrate cross- and co-polarization configurations of the ports of a common waveguide. Although not illustrated, the ports could be configured at any predetermined polarization by altering the orientation of the ports relative to each other.
FIG. 1A illustrates an antenna feed assembly 10 with cross-polarization. The assembly includes a common waveguide 12 having a first end 14 for connection to the feedhorn of an antenna, not shown. The common waveguide also includes two ports, 16 and 18, for connection to either two receive waveguides, two transmit waveguides, or a transmit and a receive waveguide. The ports, 16 and 18, are rectangular in shape so as to receive or transmit only one polarization signal. As illustrated in FIG. 1A, the first port 16 has a longitudinal dimension 16a that extends in parallel with the longitudinal axis A of the common waveguide, and the second port 18 has a longitudinal dimension 18a that extends perpendicular to the longitudinal extension A of the common waveguide. In a cross-polarization configuration, the longitudinal dimension 16a of the first port 16 and the longitudinal axis A define a first plane extending vertically in FIG. 1A substantially bisecting the common waveguide. The longitudinal dimension 18a of the second port 18 and the longitudinal axis A define a second plane extending substantially horizontally in FIG. 1A and perpendicular to the first plane. In this configuration, signals with one polarization are accepted by the first port 16, while signals with an orthogonal polarization are accepted by the second port 18. In a cross-polarization configuration, the common waveguide is typically referred to as an orthogonal mode transducer (OMT).
FIG. 1B illustrates the first and second ports in a co-polarization orientation. In this instance, the longitudinal dimension 16a of the first port 16 and the longitudinal axis A define a first plane extending horizontally in FIG. 1B, and the longitudinal dimension 18a of the second port 18 and the longitudinal axis A define a second plane extending substantially horizontally in FIG. 1B such that the first and second planes are substantially coplanar. In a co-polarization configuration, the common waveguide is typically referred to as a diplexer.
Although these antennas provide proper orientations for operating with signals that are at different polarizations, there are some current problems with the manufacture and implementation of these antennas. Specifically, signal conventions for the transmission and reception of signals may vary in different areas of the world depending on the position of satellites and possible interference between different communication signals. For example, in some areas, the received signals propagate in a horizontal plane, and the transmitted signals propagate in a vertical plane, while in other areas of the world the communication signals are oriented in an opposite configuration. In light of this, antennas must either be individually manufactured for the different signal configurations, or the antennas must be configurable in the field to select the proper configuration of the wave-guides. To decrease cost, however, it is typically preferable to manufacture one antenna that can be reconfigured in the field based on the location and the application in which it is used.
With reference to FIGS 1A and 1B, for in-field configuration, the antenna feed assembly must be rotated so as to place the ports of the waveguides in proper polarization orientation with respect to the communication signals. For example, by rotating the feed assemblies of FIGS 1A and 1B by ninety (90) degrees R the waveguides are switched in polarization. To facilitate in-field configuration, many conventional systems include a flange 20 connecting the common waveguide 12 and to the feedhorn of the antenna. During configuration, the common waveguide, as well as receiver electronics 22 and transmitter 24 connected to the common waveguide, are all rotated to the proper polarization for the application in which the antenna is used.
Although in-field configuration decreases time and cost in manufacturing, there are still drawbacks to this conventional solution. Specifically, the transmitter of an antenna is typically an expensive portion of the overall cost of the antenna. Also, given the complexity of most transmitters, they are more susceptible to damage from mishandling. Designs such as those shown in FIGS 1A and 1B that require rotation of the transmitter during in-field configuration are thus less advantageous, as it is more likely that the transmitter of the antenna can be damaged.
In addition, some new antenna designs do not allow for rotation of both of the transmitter and receiver waveguides connected to the antenna feed assembly. Specifically, the assignee of the present application has designed a new antenna that advantageously reduces the overall size of the antenna and reduces the moment forces on the support structure of the antenna. This new antenna design places the transmitter or receiver electronics on the boom arm of the antenna, as opposed to an in-line configuration behind the feedhorn, making the antenna more compact. By attaching the transmitter or receiver to the boom arm in a fixed configuration, the antenna or receiver cannot be rotated with the common waveguide to reconfigure the polarization of the antenna in the field using conventional techniques. This newly designed antenna is described in U.S. patent application Ser. No. 09/797,012, filed Mar. 1, 2001, now U.S. Pat. No. 6,417,815, and entitled: ANTENNAS AND FEED SUPPORT STRUCTURES HAVING WAVEGUIDES CONFIGURED TO POSITION THE ELECTRONICS OF THE ANTENNAS IN A COMPACT FORM, the contents of which are herein incorporated by reference.
As such, an antenna feed assembly design is needed that allows for easy in-field configuration of the polarization of the waveguides of the antenna. Further, the antenna feed assembly should allow, the feed assembly to be rotated to place the antenna in proper polarization even though one of the waveguides connected to the feed assembly is in a fixed position.
As set forth below, the present invention provides antenna feed assemblies that overcome many of the deficiencies associated with configuring the waveguides of an antenna into a proper polarization configuration. Specifically, the present invention provides antenna feed assemblies that allow the common waveguide portion of the antenna to be rotated independent of a fixed communication waveguide. When rotated, the ports of the common waveguide are altered in terms of polarization with respect to signals propagating in the common waveguide, while the predetermined polarization between the ports remains constant. A rotatable coupling between the common waveguide and the fixed communication waveguide-allows for communication of signals between the two waveguides, even though their ports are rotated with respect to each other. As such, the polarization of the waveguides associated with the antenna may be reconfigured, even though one of the waveguides remains at a fixed position.
For example, in one embodiment of the present invention, the antenna feed assembly includes a common waveguide having a body extending longitudinally between first and second ends and an opening located in the body at a point between the first and second ends. The first end of the assembly is capable of connection to a feedhorn of an antenna, and the second end is capable of connection to a fixed communication waveguide. The assembly also includes a first port in communication with the opening of the common waveguide and a second port in communication with the second end of the common waveguide. The first and second ports define respective polarizations and have a predetermined difference in polarization between each other, which may be a zero difference.
The antenna feed assembly of this embodiment further includes a rotatable coupling connected between the second end of the common waveguide and the fixed communication waveguide. This rotatable coupling allows the common waveguide to rotate with respect to the fixed communication waveguide to thereby alter the polarizations of the first and second ports associated with the common waveguide. Importantly, the rotatable coupling includes a first portion rotatably connected to a second portion. The second portion of the rotatable coupling includes a port oriented such that when the first and second portions are rotated with respect to each other, the polarization of the port of the rotatable coupling is altered with respect to the first portion of the rotatable coupling.
In use, the port of the rotatable coupling acts as an intermediary conduit for signals between the second end of the common waveguide and the fixed communication waveguide. As such, even though the first and second ports associated with the common waveguide are rotated to different polarizations, signals communicated between the second port associated with the common waveguide and the port of the fixed waveguide are properly communicated due to the port of the rotatable coupling. Specifically, if the polarization of the second port associated with the common waveguide is rotated with respect to the port of the fixed waveguide, the port of the rotatable coupling effectively rotates the polarization of the signal, such that it will be properly communicated between the second port of the common waveguide and the port of the fixed waveguide.
As mentioned above, the antenna feed assembly includes first and second ports associated with the common wave-guide. Depending on the embodiment, the second port may be an integral part of either the common waveguide or the rotatable coupling. For example, in one embodiment, the second port is an integral portion of the common waveguide and is adjacent to the second end of the common waveguide. In an alternative embodiment, the second port is an integral part of the first portion of the rotatable coupling, where it is rotatable with respect to the port located in the second portion of the rotatable coupling.
As mentioned, the rotatable coupling is positioned between the common waveguide and the fixed waveguide to allow the common waveguide to be rotated with respect to the fixed communication waveguide. In one embodiment, the second end of the common waveguide and the rotatable coupling further include flanges for mating the two together. The flanges include a pattern of openings therethrough corresponding to each other. In this embodiment, the assembly further includes fasteners extending through the openings in the flanges to retain the common waveguide and rotatable coupling in a fixed configuration. To reconfigure the polarization of the ports of the common waveguide, the fasteners are loosened so that the common waveguide is rotatable. The common waveguide, via the rotatable coupling, is then rotated through a desired angle to place the ports of the common waveguide in a new polarization orientation. The fasteners are then retightened to place the waveguide and rotatable coupling in a fixed position.
In the antenna feed assembly discussed above, the rotatable coupling of the present invention allows the common waveguide to rotate with respect to the fixed communication waveguide. In this embodiment, both the common waveguide and the antenna feedhorn are rotated. In some antenna configurations, however, it is important that the feedhorn also remain at a fixed position. Specifically, when an antenna includes a circular reflector and a circular feedhorn, the feedhorn can be rotated along with the common waveguide without offsetting the symmetry between the feedhorn and antenna. However, when the reflector is irregularly shaped, such as elliptical, rotation of the feedhorn relative to the reflector will offset the symmetry between them.
For this reason, in one embodiment, the common waveguide of the present invention further includes a flange connected to the first end for connecting the common waveguide to a flange of the feedhorn of the antenna. The flange of the common waveguide has a pattern of openings corresponding to openings in the flange of the feedhorn. The assembly further includes removable fasteners that extend through the openings in the flanges to retain the common waveguide and feedhorn in a fixed configuration.
When the common wave-guide is to be rotated, the fasteners are removed from the flange connecting the common waveguide and the feedhorn. Further, the fasteners in the flanges between the common waveguide and the rotatable coupling are loosened. The common waveguide, via the rotatable coupling, is then rotated relative to the feedhorn and the common waveguide to reconfigure the polarization orientation of the ports of the common waveguide. The fasteners are then reconnected between the flanges of the common waveguide and the feedhorn, and the fasteners between the common waveguide and the rotatable coupling are retightened to fix the common waveguide at the new position.
As discussed above, the first and second ports associated with the common waveguide are at a predetermined polarization with respect to each other to communicate signals at the proper orientation with the satellites. This predetermined difference in polarization can be any value depending on the application in which the antenna will be used. In one specific example, the common waveguide of the present invention may be an OMT. In this embodiment, the first and second ports are in a cross-polarization orientation with respect to each other with a difference in polarization of ninety (90) degrees. When rotated, the ports will remain orthogonal with respect to each other, but their polarization with respect to the signals propagating in the common waveguide will be altered.
In an alternative embodiment, the common waveguide is a diplexer in which the first and second ports are in a co-polarization orientation with respect to each other, with a difference in polarization of zero (0) degrees. When rotated, the ports will remain at the same polarization with respect to each other, but their polarization with respect to the signals propagating in the common waveguide will be altered.
In still other alternative embodiments, the first and second ports of the common waveguide are at a polarization relative to each that is at an angle other than zero (0) or ninety (90) degrees. When rotated, the ports will remain at the same polarization with respect to each other, but their polarization with respect to the signals propagating in the common waveguide will be altered.
As mentioned, the rotatable coupling of the present invention allows the common waveguide to rotate with respect to the fixed waveguide to reorient the polarization of the ports of the common waveguide. The rotation of the common waveguide can be to any angle, and in most embodiments, the rotation is an angle in the range of 0 to 90 degrees. For angles other than 0 and 90 degrees, the common waveguide will typically be circular as opposed to rectangular.
The present invention also provides an antenna that incorporates the antenna feed assembly of the present invention. The antenna includes a reflector for directing signals transmitted to or from the antenna. Extending from the reflector in a forward direction is at least one boom arm. Connected to the end of the boom arm is a feedhorn directed at the reflector for receiving and transmitting signals. Importantly, the antenna also includes a common waveguide connected to the feedhorn. The common waveguide has a body extending longitudinally between first and second ends and an opening in the body at a point between the first and second ends. Associated with the common waveguide is a first port in communication with the opening of the common waveguide and a second port in communication with the second end common waveguide. The first and second ports define respective polarizations and have a predetermined difference in polarization between each other.
The antenna also includes a fixed waveguide for communication with the feedhorn fixedly connected to the boom arm of the antenna and to the second end of the common waveguide. To rotate the common waveguide relative to the fixed communication waveguide, the antenna includes a rotatable coupling connected between the second port of the common waveguide and the fixed waveguide. The coupling allows the common waveguide to rotate with respect to the fixed waveguide to thereby alter the polarizations defined by the first and second ports while maintaining the predetermined difference in polarization between the first and second ports.